In 1918 the British Empire had been using the Lee series of bolt action battle-rifles since 1888. It was decided by the Imperial Ministry of War to take a gander at some of the more functional technologies available to militaries at the time. After approximately a years worth of research into what to pursue with further research, it was decided to go with the principles of "Gyrojet" technology. The idea that a bullet could in fact be rocket propelled.
The Imperial Ministry of War decided it was a valid pursuit due to the initial test results done by some other countries, in fact, they viewed it a valid enough reason to shove millions of pounds worth of funding into Enfield Arms Manufacturing. Enfiedl was doing research into the practicality of using a much larger cartridge than the usual .303 cartridge. As such, the end result was to investigate the potential of a round called ".505 Magnum Gibbs", the .505 round was enormous and meant for hunting large game. After bringing George Gibbs, the rounds inventor into the "fold" of the Gyrojet R&D team.
Finally, in 1920 the project truly took flight, with the newly minted round of ".505 Gyro", the round proved immensely dangerous, inf act, it's propulsion system was deemed phenomenal due to the usage of a combination ignition and propellant system within the round. The twin "rails" attached onto the shortened .505 Magnum cartridge act as a high-tensile strength friction primer for the round. The "round" itself is actually a fully-functioning rocket, small, sure, but by far one of the more lethal bullets to whizz over the heads of some Peshawar or Neo-Boxer buffoon.
The Specifications for the round are that it's tipped with a Copper and Soft-steel jacketed soft-point attached to a soft-steel/copper alloy metallurgical marvel. When the round gets the kick-start of the gigantic amount of propellant it gets lobbed down the barrel at equivalent velocity to a .500 "Tyrannosaurus" cartridge from our own timeline. With the kick-start of super-heated gasses launching it, the friction-primer's being made of an ultra-durable steel cause a grinding alongside the munition itself, acting like a striking patch on a box of matches.
After the high-velocity "match" has been struck, the round's velocity begins to drastically increase (the acceleration time can be likened to that of a.44 magnum) it flies down the rifled barrel until it finally leaves the barrel and the "nub" on the back end of the round burnt off due to the gasses and flames caused by the "kicker-charge"'s kick start. This is where the jet in gyrojet truly comes to play. It hyper-accelerates the already incredibyly fast munition towards the target, if anything, the closest comparion to it's muzzle-velocity can be compared to that of a German "Panzerbutsch" from our own timeline.
Whenever the round hits target it causes enormous tissue damage, almost like a small freight-train had a knife taped to it and given the "full-steam ahead" order. The weapon is semi-automatic, or "self-loading" as it has been called by British military instructors. The Enfield-Gibbs MKIII is fed via a ten round internal magazine, and can be reloaded either via manual feed or through stripper-clips. In order to operate the mechanism a soldier must grab the charging handle and pull back till they hear a "CLACK!", the mechanism's spring catch has just caught.
Place the stripper-clip above the opened mechanism and proceed to push down, the round will slide down into the internal feed mechanism. Proceed with tugging the charging handle once again and the bolt will smack shut, thusly becoming ready for firing. Should the weapon become jammed, there is a hydraulic shunting mechanism built within the weapon, proceed to depress the burnished steel stud underneath the barrel sticking out of the grips-fastening cap to prime the "shunt", it will via force relieve the weapon of it's jammed state should proper field maintenance have been avoided and a jam having occurred from this lack of adherence to duty. The loading system can in fact be likened to that of the Springfield Armories M1 Garand from our own timeline.
In the end, this Battle-rifle has proven to be dangerously effective, it sounds like a "Thor slaying a giant in the most gruesome of ways." so as to quote an Imperial Swedish Soldier after the second border-skirmish in South America between the two mighty nations.